Micro-phase-separated Structure and Proton Conductivity in Aromatic Polymer Electrolyte Membrane

To improve the proton conductivity of an aromatic polymer electrolyte membrane (PEM), the relationship between the size of a micro-phase-separated structure and the proton conductivity of the membrane were investigated. Micro-phase-structures, which are comprised of hydrophilic and hydrophobic domains, were measured by using the stained small-angle X-ray scattering (SAXS) method in wet conditions. Periodic sizes calculated from SAXS profiles in a dry state were as large as those measured by scanning transmission electron microscopy (STEM), which indicates that micro-phase-separated structures can be effectively evaluated by using SAXS. The membranes with the larger periodic size showed higher proton conductivity; however, water content was not simply increased with the increasing in the periodic size. Conceivably, the periodic size of the phase-separated-structure had an effect on both the proton conductivity at the same water content and on the water content itself, and the proton conductivity was increased by these two factors. The results indicated that the optimum periodic size of micro-phase-separated structures between proton conductivity and water content was 35 nm in this study, and it may be possible to develop a membrane with high proton conductivity and low water content by controlling the size of the phase-separated-structure.